The present invention relates to optical systems, devices, tools, and instruments, and to other systems having optical components and, more particularly, to a method for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time use of optical devices, and, a corresponding device and system for implementing the method thereof.
In contrast to common recreational and educational uses of optical viewing or projection devices and systems, the field of semiconductor device fabrication requires the technology of design, manufacture, and implementation of such devices and systems to be pushed to the utmost limit. Here, semiconductor devices are fabricated on silicon wafers, where a single wafer, capable of containing multiple semiconductor devices, is made up of a multiple of overlaid layers, in sequence, one on top of the other. Photolithography is an initial stage in the process of repetitively producing a single layer, involving the use of a stepper machine for optically projecting a patterned slide or mask onto a light sensitive layer or coating known as photo-resist, previously deposited onto the silicon wafer. The exposed photo-resist layer is then developed, leaving a patterned layer of photo-resist on the wafer, matching the pattern of the mask. Following completion of each layer, photo-resist can again be deposited on the wafer for forming another layer, and so on.
The continuously increasing technological requirements of complexity and speed of operation of semiconductor devices imply that wafer patterns must contain extremely fine features, on the order of a fraction of a micron wide. The design rule, or width of the finest pattern on a wafer, has significantly declined, by about a factor of 10, during the past decade. Today""s fastest devices typically feature a 0.25 micron design rule, however, new devices are currently being developed using half this size. This translates to requiring a stepper featuring an optical system to achieve a high level of optical quality such that geometrical distortion and resolution of an image are each significantly less than the design rule.
Another implication of current semiconductor technology is that features printed on one layer of a wafer must be well aligned with other features, existing underneath in preceding layers, so as to minimize alignment error between layers, commonly known as overlay or misregistration error. Maximum allowable overlay error, known as overlay budget, is about one-third of the design rule.
An overlay, or registration, metrology tool operates in conjunction with the stepper, by using a microscope for viewing patterns created by the stepper. By using the overlay metrology tool for viewing patterns of different layers and a computer for image analysis, one is able to measure the overlay error between layers. Such measurements are used for calibrating, testing, and adjusting the stepper in order to minimize the overlay error. An overlay metrology tool, however, inherently introduces its own error into the overlay measurement. This error has two components, known as the accuracy error, and the repeatability or reproducibility error. Accuracy error, also referred to as Tool Induced Shift (TIS), directly arises from distortions and aberrations in the optics of the overlay metrology tool. Repeatability error may arise from several factors, optics being one of them. Since the overlay metrology tool is used to monitor and control the stepper, tolerances placed on the total error are significantly tighter. The stepper must produce an overlay better than or within the overlay budget, however, the overlay metrology tool must in turn, produce a total error less than about one tenth of that. This tight margin of total error, especially the TIS component, translates to extremely strict requirements on the optical quality of an overlay metrology tool.
A critical dimension (CD) metrology tool is another metrology tool used for calibrating, testing, and adjusting the stepper, which is used for measuring the width of the finest lines produced by the stepper. Currently, line width tools typically feature an electron microscope, rather than conventional optical viewing systems, but the latter are still used. Other types of metrology tools featuring an optical system may also be used for making critical measurements of wafer fabrication processes.
Thus, the constant drive to increase complexity and speed of semiconductor devices invokes the tightest possible constraints and tolerances on the optics and quality of steppers and metrology tools used in wafer fabrication processes. These characteristics determine and limit the achievable complexity and speed of next generation semiconductor devices.
Another field where optical quality of optical viewing devices and systems is of extreme importance is aerial or satellite photography. Although features and objects, such as landscape and buildings, viewed by such optical equipment are relatively large, the large distances from which they are viewed result in minute details appearing in a viewed image, which can be thought of as scaling to similar conditions and dimensions involved in micro-lithography.
With respect to understanding the present invention, the following terminology and definitions are provided here, referred to and used hereinafter. An optical system refers to any system including at least one optical device, along with any number of other devices, mechanisms, units, and/or components enabling functional and cooperative operation of the optical device and the system. An optical device refers to a device, such as a tool, instrument, or piece of equipment, featuring at least one optical assembly, and at least one peripheral structure and/or at least one peripheral mechanism, positioned and/or functioning along an optical path of the at least one optical assembly for enabling viewing or projecting by the optical device.
An optical assembly features at least one optical element, and at least one peripheral structure and/or peripheral mechanism positioned and/or functioning for holding, moving, or changing the direction or orientation of the at least one optical element. An optical element is ordinarily considered as a piece of material, such as uncoated or coated glass or plastic, specially shaped to affect light rays in a specific way, including refraction, reflection, transmission, absorption, diffraction, and scattering.
Exemplary optical elements are a lens, a window or flat, a reflector or mirror, and a prism. Special types of optical elements, featuring a specially configured optical element or a combination of optical elements, include a curved mirror such as a parabolic mirror, a part-mirror, and a beam splitter, also known as a cube. A part-mirror functions by partly enabling reflection and transmission. A beam splitter features two prisms geometrically configured for splitting a beam. An optical assembly featuring at least one lens and/or at least one mirror, is commonly referred to as a lens assembly. An optical assembly, such as that featured in a particle beam microscope in general, and an electron microscope in particular, can also be of a non-conventional form such as an electric field, a magnetic field, or an electromagnetic field, serving as a lens for affecting not light in the classical form, but rather in the form of charged particles.
A peripheral structure refers to a structure peripherally positioned and functioning for holding, moving, and/or changing the direction or orientation of at least part of an optical device, such as a mount, frame, cell, tube, column, barrel, turret, eyepiece, and nosepiece. A peripheral mechanism refers to a mechanism peripherally positioned and functioning for enabling operation of at least part of an optical device, such as a source for providing electromagnetic radiation such as light or a particle beam for viewing or projecting an image. An optional peripheral mechanism is positioned and functioning for enabling optional operation of at least part of an optical device, such as a detector for detecting pixel intensities, for example, a camera for recording an image.
A microscope is an example of an optical device used for viewing and featuring the above described optical components. FIG. 1 is a schematic diagram illustrating optical components of a typical light microscope. Microscope 10 includes a plurality of optical or lens assemblies, including a condenser 12, an objective 14, and a tube-lens 16, where each lens assembly includes a plurality of optical elements or lenses 18, 20, and 22, respectively. Microscope 10 also includes another optical element, mirror 24. Light source 26 is an exemplary peripheral mechanism, whereas camera 28 is an exemplary optional peripheral mechanism of microscope 10 used for recording an image of a viewed object 30.
FIG. 2 is a schematic diagram illustrating a partial cross section of a typical microscope objective 32. As illustrated, cement 34 is used to secure an optical element, glass lens 36, in a peripheral structure, lens holder or lens cell 38. Any number of various lens cells are then assembled into some sort of peripheral structure, such as a tube 40, for holding and ultimately orienting the lens cells, forming objective 32. Mechanical axis 42 of lens cell 38, coinciding with the mechanical axis of tube 40, is shown as a reference.
Property and characteristics of circular symmetry or rotation invariance. Certain optical elements feature the property of having an optical axis, where an optical axis refers to an axis of symmetry of the optical element, whereby the optical element can be freely rotated with respect to the optical axis without affecting or changing the optical behavior of light or radiation interacting with or passing through the optical element. This property is referred to as circular symmetry or rotation invariance, and an optical element featuring this property is circularly symmetric or rotation invariant. An optical element is rotation invariant with respect to its optical axis, which is its axis of rotational symmetry.
Exemplary optical elements having an optical axis, and therefore, being circularly symmetric or rotation invariant, include lenses and mirrors. Optical elements such as a window and a flat mirror have more than one optical axis because they are flat, whereby they can be rotated with respect to any axis perpendicular to the plane of the optical element. Exemplary optical elements not having an optical axis, and therefore, not being rotation invariant, include a prism and a beam splitter. Such optical elements cannot be rotated without affecting or changing the optical behavior of light or radiation interacting with or passing through the optical element.
A single lens is one of the simplest types of optical elements in an optical assembly, which in turn, is used for forming an optical device. FIG. 3 is a schematic diagram illustrating two basic lenses, a convex lens 44 and a concave lens 46, commonly used in optical devices. Assume that each lens is perfect, having no defect such as a blemish, imperfection, or form error. An optical device, featuring a single lens 44 or 46, is extremely performance limited and of poor quality. The optical device would exhibit spherical aberration, chromatic aberration, pincushion or barrel distortion, and field curvature. Nevertheless, such an optical device features circular symmetry as a result of the presence of optical axis 48, about which either lens 44 or lens 46 is rotation invariant. Moreover, optical aberrations and distortions produced by either lens are also circularly symmetric.
Design, manufacture, and assembly of optical devices and systems. An ideal optical device or system, used for viewing or projecting a planar field, equally treats each point in the respective field of view or field of projection. All points in the respective field of view or field of projection are simultaneously focused and equally magnified, with no image distortion. In reality, such is not the case. With regard to interaction of light with matter, the laws of physics imply that, even if all basic components, such as lenses and mirrors, of an optical device or system are perfectly designed, manufactured, and assembled, the optical device or system as a whole would still deviate from ideal desired optical behavior. Dispersion of a light beam as it interacts with, and passes through, a perfectly designed, manufactured, and assembled optical device such as a lens assembly featuring a glass lens in a lens holder, is just one example of such deviant behavior.
In principle, as complexity of design of an optical device or system, and its number of optical elements increase, approach toward ideal desired optical behavior increases. This scenario is analogous to the well known mathematical technique of improving accuracy of approximating the exact form of a function by using a polynomial function with an increasing number of coefficients, where the coefficients of the function are analogous to optical elements of an optical device. The problem with designing a more complex optical device, with the goal of achieving a better theoretical optical behavior, is that as the number of optical elements featured in an optical device grows, so does the probability for introduction of additional optical defects and deviations into the optical device, originating from the manufacturing and assembly of additional optical elements.
Aside from applying theoretical principles and laws of optics, which are part of the process of designing an optical device, practical skills and art are used in manufacturing and assembling complex optical devices and systems, with the objective of minimizing the effects of optical defects and deviations introduced during the manufacture and assembly of a given complex optical device. No matter how much care and cost go into manufacturing individual optical elements of an optical device, such as glass lenses and mirrors, there are always found varying degrees of optical defects such as impurities, imperfections, and/or blemishes. Moreover, every polished glass surface has some degree of deviation from the required shape, known as form errors. In addition, individual lenses are usually covered with a coating material, such as anti-reflective coating, which introduces additional form errors and blemishes.
Following the manufacture of optical elements, there is the very elaborate and skillful task of assembling them into the final optical device. Even if all the individual optical elements have passed various quality control criteria, deviations may still be introduced into the optical device during assembly of the optical elements.
FIG. 4 is a schematic diagram illustrating various common optical deviations existing following assembly of an optical element such as a glass lens into a lens cell, in relation to the mechanical axis of symmetry 50 of the lens cell. In (A), as a reference, lens 52 is perfectly positioned in a lens cell 54, in relation to mechanical axis of symmetry 50 of lens cell 54, such that optical axis 50 of lens 52 is exactly aligned and coincides with mechanical axis of symmetry 50. In (B), lens 56 is horizontally displaced with respect to mechanical axis of symmetry 50 of lens cell 58, such that optical axis 60 of lens 56 is misaligned with respect to mechanical axis of symmetry 50. In FIG. 2C, lens 62 is angularly displaced with respect to mechanical axis of symmetry 50 of lens cell 64, such that optical axis 66 of lens 62 is misaligned with respect to mechanical axis of symmetry 50. In FIG. 2D, lens 68 is a first lens of a compound lens called a doublet, and is both horizontally and angularly displaced with respect to the second lens 70 of the doublet and with respect to mechanical axis of symmetry 50 of lens cell 72, such that optical axis 74 of lens 68 is misaligned with respect to mechanical axis of symmetry 50 of lens cell 72. In FIG. 4, lens 52 is rotation invariant, whereas, lenses 56, 62, and 68, are not rotation invariant, with respect to each of their respective lens cells.
FIG. 5 is a schematic diagram of an optical assembly 96 illustrating various common optical deviations existing following assembly of a plurality of lens cells, with each lens cell holding a lens, into a peripheral structure such as a tube or similar multiple lens cell holder, in relation to the common mechanical axis of symmetry 76 of tube 78. As a reference, lens cell 80, holding lens 82, is perfectly aligned with and coincides with common mechanical axis of symmetry 76 of tube 78, such that optical axis 76 of lens 82 is also exactly aligned and coincides with mechanical axis of symmetry 76 of lens cell 80, and with common mechanical axis of symmetry 76 of tube 78. Lens cell 84, holding lens 86, is horizontally displaced with common mechanical axis of symmetry 76 of tube 78, such that optical axis 88 of lens 86 is misaligned with respect to common mechanical axis of symmetry 76 of tube 78, even though optical axis 88 is aligned and coincides with mechanical axis of symmetry 88 of lens cell 84. Lens cell 90, holding lens 92, is angularly displaced with common mechanical axis of symmetry 76 of tube 78, such that optical axis 94 of lens 92 is misaligned with respect to common mechanical axis of symmetry 76 of tube 78, even though optical axis 94 is aligned and coincides with mechanical axis of symmetry 94 of lens cell 90.
In FIG. 5, each lens 82, 86, and 92, is rotation invariant or circularly symmetric with respect to each mechanical axis of symmetry 76, 88, and 94, respectively, but only lens 82 is rotation invariant with respect to tube 78, or with respect to optical assembly 96. Thus, as an entirety, optical assembly 96 is not rotation invariant or circularly symmetric with respect to common mechanical axis of symmetry 76. Any deviation existing following positioning a cell in tube 78 introduces further deviation in the final position of the respective lens, in additional to any deviation already incurred following mounting the respective lens into the cell as illustrated in FIG. 4. The presence of optical deviations described and shown in FIGS. 4 and 5 significantly influence any alignment process which is ordinarily performed on an optical device in order to achieve proper operation and performance of the optical device during real time viewing or projecting.
The effects of these optical defects and deviations in an optical device must be measured and monitored during quality control processes in order minimize the rejection rate of finished products, and more importantly, to prevent the release of an optical device failing required quality levels, specifications, and tolerances. Performing quality control inspection and testing on an optical device typically requires elaborate and time-consuming procedures, and even with sophisticated testing equipment and instrumentation, it is impossible to detect all defects. Manufacturing costs and time involved in rejecting finished products failing specifications are extremely high. Alternatively, instead of rejecting finished products failing the specs, a situation can arise where a manufacturer of an optical device decides to use sub-quality components, or widen the passing range of one or more quality control specifications, in order to maintain continuous operation of a production line.
The above described optical defects and deviations cause an optical device to deviate from its designed optical behavior, resulting in the occurrence of various aberrations and disturbances, such as coma and astigmatism, during real time application of the optical device. A common practice by a manufacturer of an optical device is to reduce the level of aberrations in the optical device by stopping it down using means of an aperture. However, this practice has the negative effect of reducing optical resolution of the optical device, as optical resolution is proportional to the numerical aperture (NA) of the optical device, decreasing with physical size of the aperture. Stopping down a lens assembly may be quite acceptable for recreational or educational applications of optical devices, however, typically it is highly undesirable for applications in leading edge technologies such as semiconductor fabrication.
In addition to the presence of optical defects and deviations following the cycle of design, manufacture, and assembly, of an optical device, shipment of the optical device is another way for a variety of things to go wrong. During shipment, the optical device may be exposed to mechanical shock, severe pressure change if shipped by air, and severe temperature changes. Often, characteristics and operation of an optical device, initially meeting all manufacturer quality control specifications, considerably change to the extent of being out-of-spec upon reaching the final destination of an end-user. Thus, even following a comprehensive, costly, and quality controlled cycle of design, manufacture, and assembly, an optical device may still feature defects and/or deviations at the time of application by an end-user.
In practice, an optical device, as previously described, typically includes several optical assemblies, such as lens assemblies, where one of the lens assemblies may be an objective, with each lens assembly featuring a plurality of optical elements such as lenses and/or mirrors, where each element has a varying extent of surface concavity and/or convexity, as shown in FIG. 3. Each lens and mirror may have a different radius of curvature, and may feature different glass and/or coating materials, for the purpose of attempting to correct and compensate for the presence of various degrees of optical aberrations, distortions, and field curvature of the optical device.
FIG. 6 is a schematic diagram of an optical assembly 100 illustrating an example of a plurality of lenses 102, 104, 106, and 108, having a common optical axis 110 of circular symmetry perfectly aligned and coinciding with common mechanical axis 110. Moreover, in the event that entire optical assembly 100 is rotated about common mechanical axis 110, during rotation, optical axis 110 is aligned with and coincides with rotation axis 110. Thus, according to the shown configuration of the lenses in optical assembly 100, entire optical assembly 100 is rotation invariant or circularly symmetric with respect to common mechanical axis 110.
If all the lenses and/or mirrors of an optical device, in general, and of an optical assembly or lens assembly, in particular, for example, shown in the objective of FIG. 6, were perfectly designed, manufactured, and assembled, the optical device, in general, and the optical assembly or lens assembly, in particular, would feature the property of circular symmetry and be rotation invariant. The entire optical device could be freely rotated about its optical axis of symmetry without any effect on the behavior of light or radiation interacting with the optical device. Unfortunately, this is never the case, since the components of an optical device, in general, and of an optical assembly or lens assembly, including optical elements such as lenses and/or mirrors, in particular, can never be perfectly manufactured, and assembled, for producing a perfectly functioning optical device. Moreover, the presence of optical defects and/or deviations from ideal symmetry is likely to increase with increasing sophistication in the design of an optical device, as an increasing number of optical elements, peripheral structures, and peripheral mechanisms inherently leads to the presence of additional optical defects and deviations in the optical device. There are three major contributing factors preventing the achievement of circular symmetry or rotation invariance of an optical device. The first factor relates to raw materials used for manufacturing the optical elements of an optical device. Glass or plastic, from which an optical element, capable of featuring circular symmetry, such as a lens, a mirror, and a window, is made, always has some level of impurity or blemish in it. Impurities or blemishes are randomly scattered throughout the raw material, preventing the manufacture of a uniform glass or plastic, and hence preventing the achievement of circular symmetry of the optical element, and consequently, of the optical device.
The second factor relates to the manufacturing process of the optical element, for instance polishing a precursor of a lens, a window, or a mirror, and optional coating of a precursor of a lens, a window, or a mirror. In a process of making a lens, a flat piece of glass is polished down in order to form the necessary curved surfaces of the final, usable, lens. However, such polished surfaces usually feature random locations of irregularities. In addition, in a polished lens, there always exist form errors or shape irregularities, including symmetric and/or asymmetric form errors. The common practice of plating an optical element with a coating material, such as anti-reflective coating, introduces yet additional blemishes and form errors into the optical element. The presence of these form errors and irregularities prevent the polished lens, and consequently, the optical device, from featuring a high degree of circular symmetry.
The third factor relates to aligning the optical elements as part of properly assembling an optical device. During assembly of an optical device, all the individual optical elements, capable of and preferably being circularly symmetric, need to be fully aligned with respect to the common axis of symmetry, or optical axis, of the optical device. However, as described and shown in FIGS. 2 and 3, some combination of horizontal and/or angular deviations are inevitably present in the assembled optical device, thereby preventing the entire optical device, and not only the separate optical elements, from featuring circular symmetry.
In addition to the presence of optical defects and deviations in optical assemblies and/or optical elements, factors relating to peripheral mechanisms of an optical device or system can significantly influence overall performance of the optical device or system. For example, optical devices and systems including a peripheral mechanism such as a radiation source, in general, or a light source, in particular, may be subject to further non-uniformities introduced by the source. In general, it is desirable, but practically impossible, to obtain a source that produces radiation of uniform intensity over the entire field of view, or field of projection.
Optical viewing devices and systems including an optional peripheral mechanism, such as a CCD camera, may be subject to further deviations introduced by the camera. CCD cameras contain both an optical image-sensing element as well as electronic circuitry for processing an image, both of which may introduce deviations to a recorded image.
Principle of exposure/integration time. A viewing or projection system may include one or more devices for detecting and recording images of an object. During normal operation, there is always a finite or non-zero amount of time during which a viewing or projection system is required to detect and record an image. When projecting an image, for example, the electromagnetic radiation or particle beam source passing through an image slide remains activated for a finite period of time. Similarly, when viewing an image or a series of instantaneously generated images, for example, by a camera, during the short time period of recording a picture of the series of instantaneously generated images, the recorded image is being formed for as long as the aperture of the camera remains open. This time period is commonly referred to as the exposure time. In viewing and projecting, the formed image is actually an integral of all the instantaneously generated images which are viewed by the optics during the exposure.
In the case of using an electronic charge coupled device (CCD) camera, the exposure time is also referred to as the integration time. Integration occurring during exposure time is usually considered in the art as very undesirable. When taking a picture, for example, too long of an exposure time, or, equivalently, too slow of an aperture speed, can cause blurring or smearing in images of an object. Thus, in addition to the presence of optical defects and deviations in optical elements, integration time is another factor relating to optical deviations and. overall performance of an optical device or system including imaging equipment.
There is a large variety of prior art methods, devices, and systems for eliminating, diminishing, or at least, compensating for, the effects of optical defects and deviations in optical devices and in systems including optical elements. In general, each of these involves translational and/or rotational movement of optical elements and/or optical assemblies with a goal of properly aligning or centering all such components of an optical device relative to the optical axis, while dealing with the effects of optical defects and deviations in the optical device.
In a disclosure involving rotating optical elements, but specifically relating to spatial filtering, Riggs, in U.S. Pat. No. 3,620,591 describes a method and apparatus for optically processing seismic or other data by spatial filtering in order to discriminate against optical noise and enhance recoverable information. Optical elements acting on the seismic signals are mounted in special assemblies and are rotated at different angular velocities with respect to one another. A series of partial exposures is made of the output information at selected time intervals and added to give a composite exposure.
In the Riggs disclosure, separate optical elements are mounted and separately rotated with different velocities, which places substantial mechanical complexity on such an apparatus. For optical devices comprising a large number of optical elements, especially elements which need to be tightly packed, for example, doublets and triplets wherein elements tightly touch each other, if not glued together, the disclosed method becomes impracticable. Accordingly, the method lends itself to very simple optical devices and systems, including only a few sparse elements, such as the Fourier-Transform spatial filtering projection system used by Riggs, which includes only four optical elements, each in its own optical assembly. Moreover, the fact that individual optical elements need to be separately handled, prohibits the use of proven, inexpensive and readily available mass produced off-the-shelf sealed optical assemblies such as objectives, which ordinarily cannot be taken apart for facilitating approach to the individual optical elements. The method thus forces the design, manufacturing, and assembly of totally new and unique optical assemblies, again placing substantial constraints on the practicality of the method.
Additional limitations apparent in the Riggs disclosure are that the method uses a series of separate exposures at time and rotation intervals, which is limited in its effect compared to a single prolonged exposure which better xe2x80x98smearsxe2x80x99 and diminishes the effect of optical disturbances, and there is not provided a means for aligning the mechanical axis of rotation of the individual lens mounts with the optical axis of the lens being rotated.
A general significant limitation of other prior art methods, is that they deal with effects of optical defects and deviations at the time of manufacture, assembly, and/or set-up of an optical device, for example, by translating, rotating, and aligning optical elements and/or assemblies of the optical device to an optimum configuration and performance level, immediately followed by permanently fixing, such as by cementing, the optical elements and/or assemblies, prior to release of the optical device to an end-user, or prior to use of the optical device by the end-user. In essentially all of these devices and methods, no optical assembly or optical element is rotated during regular real time use of the optical device.
In particular, some practices of manufacturing and assembling optical devices attempt to minimize effects of orientational deviations of lenses in their lens cells, as shown in FIG. 5, by procedures such as rotating one or more lens cells 84 or 90, about the common mechanical axis of symmetry 76, while slightly adjusting the orientation of the lens it holds, which is initially misaligned with respect to axis 76, by means of micro-manipulators, such that each lens axis of symmetry, optical axis 88 and 94, respectively, becomes aligned and coincides with the common mechanical axis of symmetry 76 of lens assembly 96. This procedure is commonly performed individually for all lenses of an optical assembly. Despite such advanced optical device assembly procedures, orientational deviations of lenses in lens cells can be minimized, but not entirely eliminated.
Other practices of manufacturing and assembling optical devices attempt to minimize effects of deviations of lenses in the final assembled optical device, such as the lens assembly shown in FIG. 5. One such procedure involves designating one or more lenses in a lens assembly to remain capable of slight movement, such as that caused by the force of adjustment screws, after the lens assembly has been assembled. By slightly rotating and/or translating these movable lenses with respect to the other optical elements of the lens assembly, an attempt is made to reduce the amount of optical disturbances of the entire optical assembly or device. This procedure is very limited, since simple movements of one optical element usually cannot compensate for complex disturbances originating in other optical elements of the same optical device.
In U.S. Pat. No. 5,852,518, Hatasawa et al. describe a projection optical unit for projecting a pattern on a mask, and a method for adjusting image formation of the projection unit, including relatively rotating, at least two optical elements or lenses previously and intentionally subjected to xe2x80x98astigmatic surface processingxe2x80x99, in order to correct characteristic asymmetric aberration of the overall projection optical system. Although not specifically stated, it is implied and understood by those skilled in the art, that rotating the lenses is done only during a one-time setup procedure, either during assembly, or after shipment, unpacking and installation, prior to commencing regular real time use of the projection unit The lenses are rotated according to an empirical orientation that optimizes overall behavior of the unit, and are then left fixed in place. Any additional rotation would degrade the performance of the projection unit. Nothing is rotated during actual real time use of the projection unit.
In the disclosure of Hatasawa et al., rather than alleviating asymmetry of the unit, an attempt is made to compensate for asymmetric disturbances by introducing yet additional asymmetric disturbances. The method only has benefits if some systematic and well-behaved distortion is present, which can be corrected by applying a reciprocal distortion. This clearly is not the case for the occurrence of general disturbances in optical systems, which, as previously explained, are mostly random in nature. The method is limited by not providing means for correcting various types of optical disturbances, and may even lead to introduction of new systematic disturbances. Moreover, this method can be only applied by the manufacturer of the actual projection unit, for incorporating the distorted elements into the design, production and assembly. This necessitates detailed knowledge of the design of the specific unit and is custom tailored to that unit.
In each of the following disclosures, rotation of one or more optical elements or optical assemblies, is performed exclusively during manufacturing, assembly, or under test conditions, for improving the quality of the particular optical device, prior to actual real time use of an optical device. In U.S. Pat. No. 5,835,208, Hollmann et al. discloses an apparatus and method for non-contact measuring wedge and centering errors in optical elements under test, including a lens holder rotatably supported on an air bearing having an axis such that the lens holder may be rotated about the axis. In U.S. Pat. No. 3,782,829, Herriott describes a lens alignment apparatus and method, including rotation of a lens holder about a selected optical axis until alignment is achieved, followed by permanently fixing or grounding the lens. In U.S. Pat. No. 3,762,821, Bruning et al. discloses a lens assembly apparatus and method for centering and aligning a lens element along a predetermined axis, including means of tilting the lens element about axes by using a combination of linear actuators. In U.S. Pat. No. 3,544,796, Baker discloses a lens centering apparatus including a rotatable lens holder in which the lens to be tested is permanently mounted.
In U.S. Pat. No. 2,352,179, Bosley describes a device for centering a lens on a rotating support in alignment with the axis of rotation of the support Again, rotating the lens support and the lens is performed during assembly, for improving optical quality, prior to actual application of an optical device. Included here is also the commonly known and used procedure for aligning the optical axis of a single lens to the axis of rotation. This procedure pertains to lens elements, and is only used during assembly of lens elements. When the device is fully assembled, everything is glued or otherwise fixed in place and is expected not to rotate or even move at all.
The following additional references relate to devices and methods employed exclusively for aligning optical devices, assemblies, and elements. In U.S. Pat. No. 5,400,133, Hinton et al. disclose a raster output scanner (ROS) system including a mechanism for adjusting and aligning the optical center line of scanning beams. A lens barrel has eccentric rings mounted along its circumference which, when rotated, effect movement of the lens barrel so as to change the center line of the collimated beam output. In U.S. Pat. No. 5,233,197, Bowman et al. describe a fluorescent emission imaging microscope including a galvanometer rotatable mirror placed in the sample image path, and includes an automatic objective lens piezoelectric translator for directing fine movements to achieve proper alignment. In this disclosure, the mirror is rotated for the purpose of focusing, and not for correcting optical defects and/or deviations. In U.S. Pat. No. 3,533,700, Alexander discloses a laser projection device involving coordinated orientation of at least two laser beams, including methods of optical alignment. In U.S. Pat. No. 5,453,606, issued to Hojo, an apparatus is disclosed for adjusting the optical axis of an optical system during the assembly process of a lens assembly, including automatic two-dimensional adjustment of the lenses, followed by fixing of the lenses to a lens frame.
As described above, even after applying such devices and methods for diminishing effects of optical defects and deviations, there still remains high likelihood of the presence of optical defects and deviations of the optical device by the time an end-user includes the optical device in an application. In addition, following repetitive or modified use of an initially optimally configured and performing optical device, optical defects and deviations are expected to appear, thereby limiting further application of the optical device.
To one of ordinary skill in the art, there is thus a need for, and it would be highly advantageous to have a method for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time use of optical devices, and, a corresponding device and system for implementing the method thereof.
The present invention relates to a method for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time use of optical devices, and, a corresponding device and system for implementing the method thereof.
It is therefore an object of the present invention to provide a method for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time viewing and projecting by optical devices.
It is another object of the present invention to provide a corresponding device for implementing the method of simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time viewing and projecting by optical devices.
It is another object of the present invention to provide a corresponding system for implementing the method of simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time viewing and projecting by optical devices.
Thus, according to the present invention, there is provided a method for diminishing effects of optical defects and deviations during real time use of an optical device, comprising the steps of: (a) providing an optical rotation device for rotating at least one optical part of the optical device during real time use of the optical device; and (b) rotating the at least one optical part of the optical device about a rotation axis during real time use of the optical device, by activating and controlling the optical rotation device, thereby spreading and blurring about the rotation axis the optical defects and the deviations present in the at least one optical part of the optical device.
According to another aspect of the present invention, there is provided a method for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time use of an optical device, comprising the steps of: (a) providing an optical rotation device for rotating at least one optical part of the optical device during real time use of the optical device; (b) aligning an optical axis of the at least one optical part of the optical device with a rotation axis of the at least one optical part of the optical device, causing the at least one optical part of the optical device to be circularly symmetric with respect to the rotation axis; and (c) rotating the at least one optical part of the optical device about the rotation axis during real time use of the optical device, by activating and controlling the optical rotation device, thereby circularly symmetrically spreading and blurring about the rotation axis the optical defects and the deviations present in the at least one optical part of the optical device.
According to another aspect of the present invention, there is provided a method for aligning the optical axis of at least one optical part of an optical device with a rotation axis of an optical rotation device used for rotating the at least one optical part of the optical device, comprising: (a) holding the at least one optical part by a peripheral structure of the at least one optical part, at two or more points along the peripheral structure, wherein points of projection on the optical axis of the two or more points are separated by corresponding distances along the optical axis; and (b) moving the peripheral structure held by the two or more points, such that each of the points of projection on the optical axis is moved towards the rotation axis, such that the optical axis of the at least one optical part of the optical device becomes aligned and coincident with the rotation axis of the optical rotation device.
According to another aspect of the present invention, there is provided a method for diminishing effects of optical defects and deviations during real time use of an optical device, the optical device including a light source, comprising the steps of: (a) including at least one rotation variant optical element in the optical device, such that the light source generates light rays passing through the at least one rotation variant optical element; (b) providing an optical rotation device for rotating the at least one rotation variant optical element during real time use of the optical device; and (c) rotating the at least one rotation variant optical element about a rotation axis during real time use of the optical device, by activating and controlling the optical rotation device, thereby spreading and blurring about the rotation axis the optical defects and the deviations present in the light rays of the light source passing through the at least one rotation variant optical element.
According to another aspect of the present invention, there is provided a method for diminishing effects of optical defects and deviations during real time use of an optical device, the optical device including a camera, comprising the steps of: (a) including at least one additional camera in the optical device; (b) positioning the camera and each of the at least one additional camera, such that the camera and each of the at least one additional camera faces a different direction spaced apart at equally spaced angular intervals; (c) including a rotation variant optical element in the optical device corresponding to each at least one additional camera, the rotation variant optical element is selected from the group consisting of a part mirror and a beam splitter; and (d) positioning each rotation variant optical element for diverting images toward each corresponding at least one additional camera.
According to another aspect of the present invention, there is provided an optical rotation device for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time use of an optical device, comprising: (a) a column for containing at least one optical part of the optical device; (b) a sleeve functioning as a mount for holding the column; (c) a rotation mechanism for enabling rotation of the sleeve; (d) a rotation mechanism housing for housing the rotation mechanism; (e) a motor for actuating rotation of the sleeve; (f) a transmission for enabling the motor to effect rotation of the sleeve; and (g) an adjustment mechanism for adjusting a position of the column relative to the sleeve.
According to another aspect of the present invention, there is provided an optical rotation device for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time use of an optical device, comprising: (a) a column for containing the at least one optical part of the optical device; (b) a sleeve functioning as a mount for holding the column; (c) a ring for providing slight freedom of movement required to align the column with respect to the sleeve; (d) a main rotation mechanism for enabling rotation of the sleeve; (e) a main rotation mechanism housing for housing the main rotation mechanism; (f) a motor for actuating rotation of the sleeve; (g) a transmission for enabling the motor to effect rotation of the sleeve; (h) two self-aligned rotation mechanisms positioned at either side of the main rotation mechanism; (i) pre-loaded flexures for mounting, holding, and moving the two self-aligned rotation mechanisms; and (j) two sets of actuators for actuating the pre-loaded flexures.
According to another aspect of the present invention, there is provided a system for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time viewing by an optical device, comprising: (a) the described optical rotation device; (b) an electronic control unit for activating actuator mechanisms, thereby changing positions of tie actuators, the actuator mechanisms include piezoelectric transducers; (c) a camera for recording images viewed by the optical device; (d) a digital frame grabber for capturing electronic images of the camera; and (e) a computer for controlling the electronic control unit.
According to another aspect of the present invention, there is provided a system for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time projecting by an optical device, comprising: (a) the described optical rotation device; (b) an electronic control unit for activating actuator mechanisms, thereby changing positions of the actuators, the actuator mechanisms include piezo-electric transducers; (c) a beam splitter placed in front of optics of the projecting optical device; (d) a camera for viewing images projected by the optical device; (e) a digital frame grabber for capturing electronic images of the camera; and (f) a computer for controlling the electronic control unit.
The method for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time use of optical devices, and, a corresponding device and system for implementing the method thereof, according to the present invention, provide the following advantages and benefits for real time using optical devices and systems. Improvement in the quality of an optical device or system, through reduction of asymmetrical aberrations, such as coma, reduction of distortion and astigmatism, and reduction in the effect of blemishes and impurities. This allows for better performance and tighter specifications, raising the value of the device or system. Due to elimination or reduction of said effects of optical defects and deviations of an optical device or system, the optical device or system more closely approaches its theoretical design model, becoming more predictable, allowing for designing more cost-effective optical solutions. Allows increasing the numerical aperture of the optics of an optical device or system, thus increasing optical resolution and ability to view or project finer patterns, resulting in further improving specifications and value of the optical device or system. Improves accuracy and reduces tool induced shift (TIS) in optical measurement systems such as overlay metrology tools. Improves long-term stability and reliability of optical devices and systems.
Additionally, with respect to manufacturing optical devices and systems, implementation of the present invention reduces production time and effort required to achieve tight design specifications of an optical device or system, resulting in lower production costs and shorter delivery times. Enables acceptance of components and elements of an optical device or system that otherwise would be rejected during testing processes, resulting in reduced manufacturing costs and time. Removes the risk of optical device or system failure due to effects of shipment and/or environmental conditions.
Implementation of the method of the present invention involves performing or completing selected tasks or steps manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of a given preferred embodiment of the device or system, several selected steps of the present invention could be implemented by hardware or by software on any operating system of any firmware or a combination thereof. For example, as hardware, selected steps of the invention could be implemented as a chip or a circuit. As software, selected steps of the invention could be implemented as a plurality of software instructions being executed by a computer using any suitable operating system. In any case, selected steps of the method of the invention could be described as being performed by a data processor, such as a computing platform for executing a plurality of instructions.